A Glossary of Steam

The trade’s words, defined the way the Works uses them.

Duty: The nineteenth century’s fuel-economy score: foot-pounds of work done per bushel (94 lb) of coal burned, counted in millions. Lean’s Engine Reporter published the duty of the great Cornish pumping engines monthly from 1811, and engineers chased the table like a league standing. It is the score used throughout the Works.
Indicator diagram: A plot of cylinder pressure against cylinder volume over one stroke, drawn in the period by a pencil riding the engine itself. The area inside the loop is the work of the stroke — every economy and every fault shows in its shape, which is why it is the centrepiece instrument here.
Cutoff: Closing the steam valve partway through the stroke and letting the charge finish the work by expanding. Earlier cutoff spends less steam for most of the work — the single most important economy in steam practice, and the idea that consumed engineering for a century.
Mean effective pressure (MEP): The one steady pressure that would do the same work per stroke as the real, varying pressure actually did — the indicator loop’s area averaged over the stroke. Power is MEP times area, stroke, and speed.
Atmospheric engine: The first practical steam engine, after Newcomen (1712): steam merely fills the cylinder and is condensed to make a vacuum, and the weight of the atmosphere pushes the piston down. The boiler barely works; the sky does.
Separate condenser: James Watt’s fortune (patented 1769): condense the steam in its own cold vessel instead of chilling the working cylinder every stroke. The cylinder stays hot, the vacuum stays deep, and the coal bill falls by roughly two-thirds.
Strong steam: The Cornish name for high-pressure steam, made safe-enough by Trevithick’s cylindrical boilers around 1800. Pressure pushes harder than the atmosphere ever could, so the engine shrinks until it fits on a cart — the beginning of portable and locomotive steam.
Safety valve: A weighted valve that lifts and vents the boiler at its set pressure. Screwing it down for more power was the classic road to a burst boiler — Greenwich, 1803, made the lesson famous, and the Works will happily let you repeat it.
Governor (flyball): Two spinning weights, belted off the shaft, that rise with speed and close the throttle — or, after Corliss, trim the cutoff. It is proportional only: it holds the engine within a narrow band around the set speed, never exactly upon it, and it is what stands between an unloaded engine and a burst fly-wheel.
Droop: The deliberate slope of a proportional governor: the engine settles a little above or below the set speed depending on load, because the governor needs an error to act on. A governed engine holding a shade off its mark is working correctly.
Fly-wheel: Tons of iron on the shaft that store each power pulse and repay it through the dead centres, ironing a pulsing piston into rotation a mill can trust. Every rim has a speed past which it does not stay in one piece.
Dead centre: The crank positions where the connecting rod and crank line up and the piston can exert no turning force at all. An engine parked there genuinely cannot start itself — which is why crews barred engines over.
Barring over: Levering the fly-wheel round by hand (with a bar in the barring holes) to move the crank off dead centre to an angle with strong turning effort, so steam can take hold. The Works’ “bar the engine over” button is this exact practice.
Gauge glass: The vertical glass tube on the boiler showing the water level between two marks — “the nuts.” Let the water fall out of sight and the plates over the fire soften until the shell fails; keep it brim-full and the boiler primes. The fireman’s first instrument.
Priming: A too-full or too-hard-driven boiler sending water over with the steam. Water does not compress: enough of it arriving in the cylinder meets the piston as a solid slug, and something — usually the cylinder head — gives way.
Cylinder condensation: Steam lost re-warming cylinder walls that the last exhaust just cooled — a tax on every stroke, worst in small and unlagged cylinders. Corliss’s separated hot and cold passages, compounding, and finally the uniflow were all campaigns against it.
Compound engine: An engine that expands its steam in two stages: a small high-pressure cylinder exhausts into a receiver, and a large low-pressure cylinder carries on down to the condenser. Splitting the temperature range over two cylinders halves what wall-chill can steal in each.
Receiver: The vessel between an engine’s expansion stages — a compound has one, a marine triple two — holding one cylinder’s exhaust until the next wants it. No valve sets its pressure: it floats to wherever the neighbouring cylinders’ appetites balance.
Triple expansion: The compound’s idea taken a third step: high, intermediate, and low-pressure cylinders spending the same steam in turn through two receivers, each seeing only a third of the temperature fall. On three cranks set 120 degrees apart it became the engine of the ocean liners — the Titanic’s engines were triples — and the last great form of piston steam.
Uniflow: The last idea in piston steam (Stumpf, 1908): steam enters hot at each end of the cylinder and leaves through a belt of ports at the middle, uncovered by the piston itself. Steam moves one way only, each end keeps its temperature, and one cylinder returns compound economy.
Bushel (of coal): For duty accounting, 94 pounds of coal — the customary measure the Cornish duty tables were reckoned against. Burn a bushel, count the foot-pounds, and you have your duty.